Buy a plain cotton tee, wash it three times, and look down at your left hip. The side seam that started as a straight vertical line now leaves the armpit, wanders forward, and arrives at the hem an inch or two ahead of where it began. On the other side it does the same thing in mirror image. The shirt has started to spiral around you, and nothing about your body changed between Tuesday and Friday.
Almost everybody reads this as a fit failure and buys the same shirt in another size hoping for a different result. It is neither that nor bad luck. It is a physical property of the yarn the shirt was knitted from, and it has been measured under standard test methods for decades.
Wales, courses, and the angle between them
A knitted fabric has two directions. The vertical columns of loops are wales; the horizontal rows are courses. In a fabric that behaves, a wale runs at a right angle to a course, which is what keeps a side seam hanging plumb. The trade name for the defect is spirality, and one open-access review defines it as the angle between the wale line and a line drawn perpendicular to the course line.
That review, Spirality in Knitted Fabric by H. Jamshaid, published in the Journal of Textile Science and Engineering in 2018, gathers up decades of experiments on the problem, along with the alternative names you will meet if you go looking: fabric skew and fabric torque. Skew is what the flat cloth does. Twist is what the finished garment does.
Residual torque is what the seam is obeying
Spun yarn is twisted short fibers, and twist stores energy. Let a length of sewing thread hang loose and watch it snarl back on itself. The review's conclusion across every study it surveys is blunt: the main cause is residual torque left in the yarn, which displaces loops and skews the cloth.
The evidence is specific. Work by Lord, Mohamed, and Ajgaonkar compared spinning routes and found open-end yarn produced the most spirality while twistless yarn produced almost none, having no residual torque to release. Later experiments found the angle rises with the twist multiple. Palaniswamy and Mohamed showed a two-ply yarn carrying half the twist of its singles produced none at all, because plying cancels torque instead of storing it.
Water is what lets the stored energy out. Dry yarn holds its strains in place through friction between fibers. Wet it and the water gets between the fibers, friction drops, and the locked-in stress is free to move. That is why the seam looks fine in the shop and crooked after laundering.
Cotton jersey is the worst possible case
Two ordinary properties combine badly. Cotton has higher torsional rigidity than viscose or polyester, so its yarns fight harder to untwist, and it is not thermoplastic, so a finisher cannot heat-set it to lock the loops in place. The review reports that polyester and cotton blends show less spirality than pure cotton equivalents.
Time alone does damage too. In the same body of work, fabric left stacked in a dry relaxed state for three weeks gained around seven degrees with nobody touching it. Finishing can compact a roll so it measures acceptably the day it leaves the mill, and the torque underneath waits for warehouse time and your washing machine.
Two test methods exist because buyers kept rejecting shipments
This is measured routinely before a garment reaches a shop. The American Association of Textile Chemists and Colorists publishes the methods, and its laundering test methods include two aimed at this: TM179, for skew change in fabrics after home laundering, and TM207, for seam twist in garments before and after it.
Two methods rather than one is the useful detail. A garment can twist even when the cloth it was cut from tests within tolerance, because cutting and sewing add distortion of their own. A brand that tests only the fabric can ship a shirt that fails on the body.
Seven checks before you pay
None of this is printed on a tag, so you are reading proxies. These are worth running in the shop and in the first fortnight of ownership.
- Sight the seam on the hanger. Hold the shirt by the shoulders and let it fall. If the side seam already bows forward before a single wash, that garment is starting from behind.
- Look at the hem, not the chest. Twist accumulates down the length of the body, so the bottom two inches show it first and most clearly.
- Prefer a two-ply yarn. Wording like two-ply or combed ring-spun signals a balanced construction. Where a cheap tee states nothing, assume the cheapest spinning route.
- Consider a blend for the shirts you wash most. A little polyester or a few percent of elastane tightens the fabric and holds the loops better than pure cotton jersey.
- Choose a denser structure. Interlock and pique hold their loops in a more balanced arrangement than the lightweight single jersey where the problem lives.
- Judge after the second wash, inside the return window. Buy one, launder it twice the way you actually launder things, and only then decide whether it earns a place.
- Hang or lay flat to dry. Tumble drying keeps working a fabric that is already trying to move, and it is the one habit you fully control.
One caution about the research: the surveyed experiments do not all point the same way on laundering. Some found washing and tumbling reduced the measured angle as fibers swelled and relaxed, others found wet relaxation raised it. They agree that the torque is locked in at spinning and that water releases it. Which way your shirt goes is not predictable from a label.
What a tailor can and cannot do about it
Very little, which is worth knowing before you pay for an alteration. A twisted side seam on a knit is not a seam-allowance problem, so unpicking and restitching puts the same skewed cloth back in the same place, and pressing does nothing because the fabric is not creased but wrongly oriented. On a woven garment cut off grain a tailor can recut a panel true; on a jersey tee the labour costs more than the shirt. The practical answer is to demote it to a layering shirt, where a drifting seam is hidden by a jacket and the cloth is otherwise sound.
Mark the seam on your next new shirt
Do this once and stop guessing. When a new jersey top comes home, lay it flat, line the side seams against the edge of a table, and take a phone photo. Wash it twice, lay it out in the same spot, and take the second photo. The difference between the two images is the residual torque, made visible in your own machine and water.
Run that on three or four brands and a pattern appears fast: some labels and weights hold their line, and very cheap single jersey does not, whatever the fiber content on the neck tape says. Two photographs give you a buying list that a size chart cannot.